ABC transporters are a group of membrane transporter proteins that play a major role in the transport and protection of cells against a wide variety of pharmacological agents, potentially toxic drugs, and xenobiotics. ABC transporters are homologous membrane proteins that bind and use cellular adenosine triphosphate (ATP) for their specific activities. Some of these transporters were discovered as multidrug resistance proteins (like the MDR1-P glycoprotein, or the multidrug resistance protein, MRP1), defending malignant cancer cells against chemotherapeutic agents. Up until the present time, 48 Human ABC Transporters have been identified, and these have been arranged into 7 families based on their sequence identity and function.
ABC transporters play a variety of important physiological roles within the body, as well as providing a defense against harmful compounds from the environment. Moreover they represent important potential drug targets both in their own right, as well as, because in many cases therapeutic drugs are also transported out of the target cell by these molecules.
One of the members of the ABC transporter family, namely, CFTR, is believed be the chloride channel responsible for cAMP-mediated chloride secretion in epithelial cells, and to play a key role in the secretion of chloride and maintenance of normal electrolyte transport throughout the body. CFTR is a protein of approximately 1480 amino acids made up of two repeated elements, each comprising six transmembrane segments and a nucleotide-binding domain. The two repeats are separated by a large, polar, regulatory (R)-domain containing multiple potential phosphorylation sites.
The gene associated with CFTR has been identified and sequenced (See Gregory, R. J. et al. (1990) Nature 347:382-386; Rich, D. P. et al. (1990) Nature 347:358-362), (Riordan, J. R. et al. (1989) Science 245:1066-1073). A defect in this gene leads to cystic fibrosis (hereinafter “CF”), the most common fatal genetic disease in humans affecting approximately one in every 2,500 infants born in the United States. Within the general United States population, up to 10 million people carry a single copy of the defective gene without apparent ill effects. In contrast, individuals with two copies of the CF associated gene suffer from the chronic effects of CF, including chronic lung destruction and death.
In patients with CF, expression of the CF associated gene in airway cells, leads to reduced cellular apical chloride conductance causing an imbalance in ion and fluid transport. It is widely believed that this leads to the abnormal mucus secretion in pancreatic ductules and in the airways that ultimately results in the pulmonary infections and epithelial cell damage typically associated with disease progression in CF. In addition to respiratory problems, CF patients typically suffer from gastrointestinal problems, and pancreatic insufficiency. Males are almost uniformly infertile and fertility is decreased in females. In contrast to the severe effects of two copies of the CF associated gene, individuals with a single copy of the CF associated gene exhibit increased resistance to cholera and to dehydration resulting from diarrhea—perhaps explaining the relatively high frequency of the CF gene within the population.
Sequence analysis of the CFTR gene of CF chromosomes has revealed a variety of disease causing mutations (Cutting, G. R. et al. (1990) Nature 346:366-369; Dean, M. et al. (1990) Cell 61:863:870; and Kerem, B-S. et al. (1989) Science 245:1073-1080; Kerem, B-S et al. (1990) Proc. Natl. Acad. Sci. USA 87:8447-8451). At present, more than 1000 mutations in the CF gene have been identified (http://www.genet.sickkids.on.ca/cftr/), but population studies have indicated that the most common CF mutation, a deletion of the 3 nucleotides that encode phenylalanine at position 508 of the CFTR amino acid sequence, is associated with approximately 70% of the cases of cystic fibrosis. The mutated CFTR protein is referred to as ΔF508.
It is believed that the deletion of residue 508 in ΔF508-CFTR prevents the nascent protein from folding correctly, resulting in the inability of this mutant protein to exit the endoplasmic reticulum (hereinafter “ER”), and traffic to the plasma membrane. As a result, insufficient amounts of the mature protein are present at the plasma membrane and chloride transport within epithelial tissues is significantly reduced (Quinton, P. M. (1990), FASEB J. 4: 2709-2727). Hence, the cellular phenomenon of defective ER processing of other proteins like CFTR, by the ER machinery, has been shown to be the underlying basis for a wide range of isolated and inherited diseases. The two ways that the ER machinery can malfunction is either by loss of coupling to ER export of the proteins leading to degradation, or by the ER accumulation of these defective/misfolded proteins [Aridor M, et al., Nature Med., 5 (7), pp 745-751 (1999); Shastry, B. S., et al., Neurochem. International, 43, pp 1-7 (2003); Rutishauser, J., et al., Swiss Med Wkly, 132, pp 211-222 (2002); Morello, J P et al., TIPS, 21, pp. 466-469 (2000); Bross P., et al., Human Mut., 14, pp. 186-198 (1999)]. Studies have shown, however, that ΔF508-CFTR, when presented at the plasma membrane is functional as a cAMP-responsive Cl− channel (Dalemans et al. (1991), Nature Lond. 354:526-528; Denning et al., supra.; Pasyk and Foskett (1995), J. Cell. Biochem. 270: 12347-50).
Although CFTR transports a variety of molecules in addition to anions, this role of transporting anions represents an important element in the overall cellular machinery for transporting ions and water across the epithelium. The other elements include the epithelial Na+ channel, ENaC, Na+/2Cl−/K+ co-transporter, Na+—K+-ATPase pump and the basolateral membrane K+ channels, that are responsible for the uptake of chloride into the cell.
These elements work together to achieve directional transport across the epithelium via their selective expression and localization within the cell. Chloride absorption takes place by the coordinated activity of: (i) ENaC and CFTR present on the apical membrane; and (ii) the Na+—K+-ATPase pump and Cl− channels expressed on the basolateral surface of the cell. Secondary active transport of chloride from the luminal side leads to the accumulation of intracellular chloride, which can then passively leave the cell via Cl− channels, resulting in a vectorial transport. Arrangement of Na+/2Cl−/K+ co-transporter, Na+—K+-ATPase pump and the basolateral membrane K+ channels on the basolateral surface and CFTR on the luminal side coordinate the secretion of chloride via CFTR on the luminal side. Because water is probably never actively transported itself, its flow across epithelia depends on tiny transepithelial osmotic gradients generated by the bulk flow of sodium and chloride.
In addition to CF, modulation of CFTR activity may be beneficial for other diseases not directly caused by mutations in CFTR, such as secretory diseases and other protein folding diseases mediated by CFTR. These include, but are not limited to, chronic obstructive pulmonary disease (hereinafter “COPD”), dry eye disease, and Sjögren's Syndrome.
COPD is characterized by airflow limitation that is progressive and not fully reversible. The airflow limitation is due to mucus hypersecretion, emphysema, and bronchiolitis. Activators of mutant or wild-type CFTR offer a potential treatment of mucus hypersecretion and impaired mucociliary clearance that is common in COPD. Specifically, increasing anion secretion across CFTR may facilitate fluid transport into the airway surface liquid to hydrate the mucus and optimized periciliary fluid viscosity. This would lead to enhanced mucociliary clearance and to a reduction in the symptoms associated with COPD. Dry eye disease is characterized by a decrease in tear aqueous production and abnormal tear film lipid, protein and mucin profiles. There are many causes of dry eye, some of which include age, Lasik eye surgery, arthritis, medications, chemical/thermal burns, allergies, and diseases, such as CF and Sjögrens's syndrome. Increasing anion secretion via CFTR would enhance fluid transport from the corneal endothelial cells and secretory glands surrounding the eye to increase corneal hydration. This would help to alleviate the symptoms associated with dry eye disease. Sjögrens's syndrome is an autoimmune disease in which the immune system attacks moisture-producing glands throughout the body, including the eye, mouth, skin, respiratory tissue, liver, vagina, and gut. Symptoms include dry eye, mouth, and vagina, as well as lung disease. The disease is also associated with rheumatoid arthritis, systemic lupus, systemic sclerosis, and polymypositis/dermatomyositis. Defective protein trafficking is believed to cause the disease for which treatment options are limited. Modulators of CFTR activity may hydrate the various organs afflicted by the disease and help to elevate the associated symptoms.
As discussed above, it is believed that the deletion of residue 508 in ΔF508-CFTR prevents the nascent protein from folding correctly, resulting in the inability of this mutant protein to exit the ER, and traffic to the plasma membrane. As a result, insufficient amounts of the mature protein are present at the plasma membrane and chloride transport within epithelial tissues is significantly reduced. In fact, this cellular phenomenon of defective ER processing of ABC transporters by the ER machinery, has been shown to be the underlying basis not only for CF disease, but for a wide range of other isolated and inherited diseases. The two ways that the ER machinery can malfunction is either by loss of coupling to ER export of the proteins leading to degradation, or by the ER accumulation of these defective/misfolded proteins [Aridor M, et al., Nature Med., 5 (7), pp 745-751 (1999); Shastry, B. S., et al., Neurochem. International, 43, pp 1-7 (2003); Rutishauser, J., et al., Swiss Med Wkly, 132, pp 211-222 (2002); Morello, J P et al., TIPS, 21, pp. 466-469 (2000); Bross P., et al., Human Mut., 14, pp. 186-198 (1999)].
The diseases associated with the first class of ER malfunction are CF (due to misfolded ΔF508-CFTR), hereditary emphysema (due to α1-antitrypsin; non Piz variants), hereditary hemochromatosis, coagulation-fibrinolysis deficiencies, such as protein C deficiency, Type 1 hereditary angioedema, lipid processing deficiencies, such as familial hypercholesterolemia, Type 1 chylomicronemia, abetalipoproteinemia, lysosomal storage diseases, such as I-cell disease/pseudo-Hurler, mucopolysaccharidoses (due to lysosomal processing enzymes), Sandhof/Tay-Sachs (due to β-hexosaminidase), Crigler-Najjar type II (due to UDP-glucuronyl-sialyc-transferase), polyendocrinopathy/hyperinsulemia, Diabetes mellitus (due to insulin receptor), Laron dwarfism (due to growth hormone receptor), myleoperoxidase deficiency, primary hypoparathyroidism (due to preproparathyroid hormone), melanoma (due to tyrosinase). The diseases associated with the latter class of ER malfunction are glycanosis CDG type 1, hereditary emphysema (due to α1-antitrypsin (PiZ variant), congenital hyperthyroidism, osteogenesis imperfecta (due to Type I, II, IV procollagen), hereditary hypofibrinogenemia (due to fibrinogen), ACT deficiency (due to α1-antichymotrypsin), Diabetes insipidus (DI), neurophyseal DI (due to Vasopressin hormone/V2-receptor), neprogenic DI (due to aquaporin II), Charcot-Marie Tooth syndrome (due to Peripheral myelin protein 22), Perlizaeus-Merzbacher disease, neurodegenerative diseases such as Alzheimer's disease (due to βAPP and presenilins), Parkinson's disease, amyotrophic lateral sclerosis, progressive supranuclear plasy, Pick's disease, several polyglutamine neurological disorders such as Huntington, spinocerebullar ataxia type I, spinal and bulbar muscular atrophy, dentatorubal pallidoluysian, and myotonic dystrophy, as well as Spongiform encephalopathies, such as hereditary Creutzfeldt-Jakob disease (due to Prion protein processing defect), Fabry disease (due to lysosomal α-galactosidase A) and Straussler-Scheinker syndrome (due to Prp processing defect).
In CF, chloride transport mediated by the CFTR is reduced resulting in the abnormal mucus secretion that characterizes the disease. By contrast in secretory diarrheas epithelial water transport is dramatically increased as a result of secretagogue activated chloride transport. The mechanism involves elevation of cAMP and stimulation of CFTR.
Although there are numerous causes of diarrhea, the major consequences of diarrheal diseases, resulting from excessive chloride transport are common to all, and include dehydration, acidosis, death and impaired growth.
Acute and chronic diarrheas represent a major medical problem in many areas of the world. Diarrhea is both a significant factor in malnutrition and the leading cause of death (5,000,000 deaths/year) in children less than five years old.
Secretory diarrheas are also a dangerous condition in patients of acquired immunodeficiency syndrome (AIDS) and chronic inflammatory bowel disease (IBD). Sixteen million travelers to developing countries from industrialized nations every year develop diarrhea, with the severity and number of cases of diarrhea varying depending on the country and area of travel.
Diarrhea in barn animals and pets such as cows, pigs and horses, sheep, goats, cats and dogs, also known as scours, is a major cause of death in these animals. Diarrhea can result from any major transition, such as weaning or physical movement, as well as in response to a variety of bacterial or viral infections and generally occurs within the first few hours of the animal's life.
The most common diarrheal causing bacteria is enterotoxogenic E-coli (ETEC) having the K99 pilus antigen. Common viral causes of diarrhea include rotavirus and coronavirus. Other infectious agents include cryptosporidium, giardia lamblia, and salmonella, among others.
Symptoms of rotaviral infection include excretion of watery feces, dehydration and weakness. Coronavirus causes a more severe illness in the newborn animals, and has a higher mortality rate than rotaviral infection. Often, however, a young animal may be infected with more than one virus or with a combination of viral and bacterial microorganisms at one time. This dramatically increases the severity of the disease.
Accordingly, there is a need for modulators of an ABC transporter activity, and compositions thereof, that can be used to modulate the activity of the ABC transporter in the cell membrane of a mammal.
There is a need for methods of treating ABC transporter mediated diseases using such modulators of ABC transporter activity.
There is a need for methods of modulating an ABC transporter activity in an ex vivo cell membrane of a mammal.
There is a need for modulators of CFTR activity that can be used to modulate the activity of CFTR in the cell membrane of a mammal.
There is a need for methods of treating CFTR-mediated diseases using such modulators of CFTR activity.
There is a need for methods of modulating CFTR activity in an ex vivo cell membrane of a mammal.
There is a need for modulators that enhance the activity and/or function of CFTR in the plasma membrane.